As a result of the Comprehensive Test Ban Treaty (CTBT), monitoring stations have been established by the Comprehensive Test Ban Treaty Organization (CTBTO) as part of an International Monitoring System (IMS) that incorporates seismic, acoustic, and radionuclide monitoring processes to detect nuclear detonations. Some such monitoring stations are configured to continuously monitor air for the detection of fission product radioactive isotopes of xenon (Xe) (also referred to as “radioxenon”). The detection of radioxenon in the air definitively confirms that nuclear fission has occurred. To ensure the accuracy of the systems (e.g., beta-gamma coincidence counting systems) employed at the monitoring stations to detect radioxenon, the detectors used by the systems are regularly calibrated for detection efficiency.
Some methods of detector efficiency calibration utilize a calibration standard separate from a gas sample to be assayed. However, such efficiency calibrations are only valid where the calibration standard has substantially the same geometric configuration and substantially the same activity as the gas sample to be assayed. It can be difficult to procure a calibration standard in the exact geometric configuration used for a gas sample to be assayed, and transferring a calibration standard to the geometric configuration of the gas sample usually voids the certified value of the standard. In addition, a calibration standard can suffer from uncertainties in its certified value propagated from the sum total of the uncertainties in the various measurements made during the production of the calibration standard, as well as uncertainties associated with the fitting (e.g., polynomial fitting) observed activities of the calibration standard (e.g., if the calibration standard includes multiple radioactive isotopes).
Other methods of detector efficiency calibration involve absolute efficiency calibration (AEC) through coincidence counting to determine absolute activity and absolute detection efficiency through the comparison of coincidence and anti-coincidence events with the known decay branching ratio data. Such calibration methods can be effectuated without the use of a calibration standard separate from a gas sample being assayed by the system. A quantified gas sample including a radioxenon isotope (e.g., xenon-133) is typically directed into an opening (e.g., a well) in a detector (e.g., a gamma ray well detector) lined (e.g., coated) with a porous solid organic scintillator, and is then subjected to AEC analysis to determine the absolute activity of the quantified gas sample and the absolute detection efficiency of the detector. Unfortunately, a portion of the quantified gas sample can become irremovably trapped within the pores of the porous solid organic scintillator, resulting in the destruction of the quantified gas sample, as well as detector memory effects effectuated by the trapped portion of the quantified gas sample. The memory effects can usually be corrected for to allow the detector to be used for subsequent gas sample assay, but such corrections can undesirably require implementing increased radioxenon detection limits.
It would, therefore, be desirable to have new systems, methods, devices, and structures for assaying a radioactive gas (e.g., to determine the absolute activity of the radioactive gas, and detection efficiencies of employed radiation detector(s)), such as gas including one or more radioxenon isotopes, that are more efficient, more accurate, and/or more versatile as compared to conventional systems, conventional methods, conventional devices, and conventional structures.